600 



Comparative Animal Physiology 



of the slow system elicits no propagated spikes but only e.p.p.'s; although 

 as the frequency increases spikes do appear. When the "fast" system is stimu- 

 lated, propagated impulses are set up by single shocks and a fast "twitch" 

 occurs. When the "slow" system is stimulated at low frequencies, only local 

 end-plate potentials are recorded and local contractions occur, facilitation is 

 marked; the e.p.p. grows as impulses arrive, some propagated spikes appear 

 at high frequencies and the contraction grows proportionately more than 

 the e.p.p. The e.p.p.'s and associated local contractions are more important 

 than they are in frog muscle because of the much greater number of nerve 

 endings, and the sign and shape of a potential recorded from the muscle 

 surface depends on the orientation of the active recording electrode with 

 respect to the loci of the e.p.p.'s. Stimulation of an inhibitory axon'^*^ has 

 two effects: (1) reduction in the e.p.p. and consequent suppression of spikes 

 and local contractions, the inhibitory state persisting 20-25 msec, and (2) 

 abolition of the local mechanical response without affecting the e.p.p. Once 

 a propagated impulse is started there can be neither electrical nor mechanical 

 inhibition. The electrical inhibition resembles the action of curare in re- 

 ducing e.p.p.'s of frog muscle. The mechanical inhibition can summate even 

 at frequencies as low as 10 per second. 



The motor axons differ in the size of e.p.p. produced and in facilitation. 

 It can be postulated that the "fast" and "slow" fibers liberate different amounts 

 of "exciting" transmitter; however, all attempts to identify chemical mediators 

 in crustacean neuromuscular systems have failed (see p. 614). A modified 

 schematic representation"-"' '-'^^ of the transmission system follows: 



Motor Impulse 



"Slow" fiber 



"Fast" fiber 



Electrical 

 Inhibition 



Muscular 

 Inhibition 



"local" 



contraction 



much 



facilitation 



no facilitation 

 e.p.p. large 



propagated spike 

 "Twitch contraction 



Inhibition is indicated by blocks; broken lines indicate that in many muscles 

 the "fast" and "slow" responses intergrade according to frequency of stimula- 

 tion. 



